An object reference is required to access non static member

using System.Collections;
using System.Collections.Generic;
using UnityEngine.UI; 
using UnityEngine;
 
public class fire_damage : MonoBehaviour {
    public int f_damage;
    // Use this for initialization
    void Start () {
        
    }
    
    // Update is called once per frame
    void OnTriiferEnter (Collider col) 
    {
        if (col.tag == "Player") {
            player_health.damage = player_health.damage + f_damage;
        }   
    }
}

using System.Collections;
using System.Collections.Generic;
using UnityEngine.UI;
using UnityEngine;
 
public class player_health : MonoBehaviour {
    public int hp_max;
    public int hp;
    public Slider slide;
    public int damage;
    // Use this for initialization
    void Start () {
        hp_max = 100;
        hp = hp_max;
        damage = 0;
    }
 
    // Update is called once per frame
    void Update () {
        slide.value = hp;
            hp = hp - damage;
        
    }
}

public class player_health : MonoBehaviour { 
 
    public static int damage;
 
}

public class fire_damage : MonoBehaviour {
 
    void OnTriiferEnter (Collider col) {
        if (col.tag == "Player") {
            var ph = col.gameObject.GetComponent<player_health>();
            ph.damage += f_damage;
        }   
    }
 
}